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Exercise 4A [Pre-Lab]

Measuring BiodiversityParts of this lab adapted from General Ecology Labs, Dr. Chris Brown, Tennessee Technological University and Ecology onCampus, Dr. Robert Kingsolver, Bellarmine University.

In this lab exercise, we will examine several concepts related to biodiversity. The termbiodiversity describes the number and abundance of species inhabiting a community, habitat orother described area. The most basic measure of biodiversity is species richness (S), a totalcount of species present in the defined area. However, species richness does not give acomplete picture of biodiversity. In this exercise, we will examine not only species richness, butalso methods to determine relative abundance (the proportion of each species present),species evenness (how equal in abundance species are) and calculate the Shannon Index, ameasure of biodiversity.

IntroductionSo, how many species are there on the planet? This question is easier to ask than to answer,because we have discovered and described only a fraction of the earth's biota. Most largeterrestrial organisms, such as birds and mammals, are so well inventoried that discovery of anew species in these taxonomic groups is a newsworthy event. At the other extreme are soilbacteria, often impossible to culture in standard media, and so poorly studied that there areprobably a number of undescribed species thriving beneath your campus grounds. In hisinfluential book The Diversity of Life, ecologist E. O. Wilson (1999) reports an estimate of 1.4million described species, based on interviews with taxonomists specializing in a wide variety oforganisms, and comprehensive reviews of databases and museum records. As for the numbersof living species not yet known to science, estimates range from 5 million to 100 million. Asecologists discover new taxonomic categories and new microhabitats, they are constantlyrevising their estimates. Wilson's argument for more attention to taxonomic questions in biologyis compelling, since intelligent management decisions for global species protection begin withsome idea of the number of species we have to protect.

How can we improve our appraisal of biodiversity yet to be discovered? One practical approachis based on repeated sampling of a type of organism in a particular place, using the growingdatabase to develop a species accumulation curve. To show how this works, let's visit LaSelva, Costa Rica's national rainforest preserve. Here entomologists John T. Longino andRobert Colwell have been collecting ants from the leaf litter of the forest floor in an extendedsurvey of the insect fauna of the park. One of their methods for trapping specimens is theBerlese apparatus. Leaf litter collected from the forest floor is returned to a lab and placed in afunnel lined with screening. A lightbulb placed over the top of the funnel heats up the leaf litter,and all the tiny invertebrates from that sample go down through the screening and into thefunnel. The bottom of the funnel leads to a jar of preservative, so the species in this sample canbe counted and identified.

Longino and Colwell's Berlese results for La Selva are shown in Figure 4.1. The x-axis showsthe number of samples analyzed, and the y-axis shows cumulative numbers of ant speciesfound in all the samples up to that point. The curve rises steeply at first, because new speciesare discovered in nearly every sample at the beginning of the study. As more and more

84 Exercise 4A. Measuring Biodiversity [pre-lab]

Number of samples

samples are examined, it becomes harder and harder to find species not already counted, sothe slope of the curve gets less and less steep as the sampling effort continues. From a quickexamination of the figure, you could predict that biologists could find more ant species if thestudy was continued, but you could also place an upper limit on the expected number, basedon the decreasing slope of the curve. Species accumulation curves have been developed formany kinds of organisms in many kinds of habitats. The shape of the curve varies somewhat,depending on habitat patchiness and the relative frequency of rare species, but declining ratesof return on sampling effort are common to all. At some arbitrary stopping point, say less thanone new species per 1000 samples, we can conclude for all practical purposes that the fauna inthis locale have been adequately described.

In another example of a species abundance curve (Figure 4.2), the cumulative number ofspecies is plotted against the total number of organisms observed (instead of the number ofsamples). You should note two things about these graphs. First, the y-axis counts cumulativenumber of species; that is, for each new sample you add only new species to the alreadyexisting total. Second, the curve flattens out as you increase the number of samples taken. InFigure 4.2, for example, you only have to look at about 22 organisms to reach 10 distinctspecies. However, to add another 10 distinct species you have to look at an additional 80 or soindividuals (that is, you don’t get to 20 total species until you’ve gone through about 105individuals). This indicates that, initially, you find a lot of new species, but as you go along thechances of finding something new gradually decreases, since in later samples most of theorganisms you collect have previously been collected.

Figure 4.1 Species Accumulation curve for antscollected with Berlese trapping in La Selva BiologicalStation in Costa Rica.; cumulative species vs. numberof samples.

Figure 4.2 Example of Species Accumulation curvewith cumulative species vs. total number oforganisms observed.

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A second way to illustrate our results is to construct a graph of proportional abundance versusrank (with rank = 1 being the most abundant, rank =2 being second most abundant, and so on),which we call a rank-abundance curve.

You’ll note that all rank-abundancecurves decline as the rank increases, butsome do so more steeply than others.For example, the curve labeled“Bromilead, Rain” goes down much morerapidly than does, for instance, the curve“Cup, Rain.” This tells you that the“Bromilead, Rain” community is lesseven than the “Cup, Rain” community; inother words, the bromeliad community isdominated by a few common species,while the cup community has a numberof equally common species.

An Example of Evaluating BiodiversitySuppose ecology students at two hypothetical colleges held a contest to see which had themost biologically diverse bird community (or avifauna) on their campus. College A studentsspent a Saturday morning walking the campus, counting numbers of each species theyencountered. Students at College B engaged in a similar sampling routine. Assume that bothcolleges had the same species richness (five species) and the same sample size of 100 birds,but that numbers of each kind of bird were distributed differently. (For a visual representation ofthe two campus data sets, see Figure 4.4.)

Type of Bird Campus A Campus B

Pigeon 96 20Robin 1 20Starling 2 20Purple Grackle 1 20House Sparrow 1 20

Figure 5.3 Rank-Abundance Curve

Table 4.1 Number of Birds Species Identified on Two Campuses

Figure 4.3 Rank-Abundance Curve

86 Exercise 4A. Measuring Biodiversity [pre-lab]

Which school has the more diverse avifauna? Clearly, four of the birds on Campus A's specieslist make little contribution to the bird community. Campus B's sample, illustrating maximumevenness, demonstrates greater biodiversity. On Campus B, interactions between differentspecies are much more frequent because the community has no single dominant type. Anotherway of thinking about this as an observer is to ask, what is the probability of encountering thesame species twice in a row? On Campus A, the probability is very high, but on Campus B it ismuch lower.

This probability of change in encounters was used along with species richness to design a wayto measure biodiversity that takes both numbers of species and their proportions into account.Called a Shannon diversity index, this methodology measures the likelihood of repetition inadjoining samples.

The diversity index is calculated as follows:

To calculate the index, first divide the number of individuals of species #1 you found in yoursample by the total number of individuals of all species. This is P1, which should be expressedas a decimal value between 0 and 1. Then multiply this fraction times its own natural logarithm.This gives you the quantity (P1 * In P1 ). Since the natural log of a fraction yields a negativenumber, a minus sign is placed in front of the parentheses in the equation to convert thenegative product back to a positive number. Next, plug in species # 2 numbers to calculate– (P2 * In P2). Repeat for all species through the last on your species list, which is speciesnumber 5. Finally, sum the - (Pi * In Pi) products for all species to get the value of the index, H.

Table 1

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The following table demonstrates calculations of the diversity indices for the bird data onCampus A and Campus B.

CAMPUS A BIRDS Ni Pi In Pi – (Pi· In Pi)

Pigeon 96 .96 -.041 .039

Robin 1 .01 -4.61 .046

Starling 1 .01 -4.61 .046

Purple grackle 1 .01 -4.61 .046

House sparrow 1 .01 -4.61 .046

TOTAL 100 H = 0.223

CAMPUS B BIRDS Ni Pi In Pi – (Pi· In Pi)

Pigeon 20 .20 -1.61 0.322

Robin 20 .20 -1.61 0.322

Starling 20 .20 -1.61 0.322

Purple grackle 20 .20 -1.61 0.322

House sparrow 20 .20 -1.61 0.322

TOTAL 100 H = 1.610

High values of H represent more diverse communities. A community of only one species wouldhave an H value of 0, since Pi would equal 1.0, and it would be multiplied by In (1.0) = O.Campus A's H value is small, because its community is dominated by one species. If allspecies are equal in numbers, the equation yields a maximum H value equal to the naturallogarithm of the number of species in the sample. For example, Campus B has five species.The H value of 1.61 = In (5), so Campus B is as diverse as a five-species community canpossibly be.

Table 4.2 Calculation of Shannon Index for Campus A and Campus B

88 Exercise 4A. Measuring Biodiversity [pre-lab]

? Check your progressWhat added information about biodiversity does the Shannon index convey that could not bederived from a simple species count?

In addition to species richness and the Shannon Index, there is a measure called speciesevenness, which attempts to measure the equality of the abundances for each species. Theformula for evenness is given below:

Shannon evenness index: J = H / ln(S) Where:J = Shannon Evenness IndexH = Shannon IndexS = species richness (# of species)ln = natural log

Using our values calculated above, we get J = 0.139 for Campus A, and J = 1 for Campus B(try it and see!). J has a maximum value of 1, which occurs when all species have equalabundances. Thus, we can conclude that Campus B has greater evenness than Campus A.